Note pubs.acs.org/Organometallics
Derivatization of Preformed Platinum N‑Heterocyclic Carbene Complexes with Amino Acid and Peptide Ligands and Cytotoxic Activities toward Human Cancer Cells Edith Chardon,†,‡ Georges Dahm,† Gilles Guichard,*,‡ and Stéphane Bellemin-Laponnaz*,† †
Institut de Physique et Chimie des Matériaux de Strasbourg, Université de Strasbourg-CNRS UMR 7504, 23 Rue du Loess, BP 43, 67034 Strasbourg, France ‡ Institut Européen de Chimie et Biologie, Université de Bordeaux-CNRS UMR5248, 2 Rue Robert Escarpit, 33607 Pessac, France S Supporting Information *
ABSTRACT: A simple procedure for the preparation of Nheterocyclic carbene platinum complexes with a nitrogenbased neutral ligand in trans geometry is presented. The lability of a trans pyridine ligand in an N-heterocyclic carbene− Pt(II)−pyridine complex, namely, (3-benzyl-1methylimidazolylidene)PtI2(pyridine), 2, was probed by a displacement reaction with various nitrogen-based ligands (e.g., amines, hydrazine, amino esters, and peptides) to yield the corresponding complexes, which could be easily isolated by column chromatography. Two representative complexes could be characterized by X-ray crystallographic studies. This strategy allows generating diversity in metallodrug candidates. Preliminary results of the biological effects on various human cancer cells and comparison wih cisplatin are reported.
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INTRODUCTION Biomedical inorganic chemistry has become an important area of chemistry in the past half century, and its contributions to medicinal sciences remain numerous, ranging from the development of diagnostic imaging agents to chemotherapeutics. The interface between medicine and inorganic chemistry has been illuminated by the development of cisplatin, one of the most effective anticancer drugs that is used to treat many forms of cancers. Today, however and even though metal-based pharmaceuticals are gaining in importance,1 especially in cancer therapies, progress in selective targeting of cancer cells still needs to be achieved. The effectiveness of cisplatin and its marketed derivatives (e.g., carboplatin and oxaliplatin) is limited by severe side effects and by tumor resistance to the drug intrinsically or over time.2 One current approach to address these pharmacological issues is to fine-tune the properties of the metal-based agents by introducing tailored multifunctional ligands.3 Recently, N-heterocyclic carbenes (NHCs) have attracted considerable interest as new ligands for the design of metalbased chemotherapeutic agents.4,5 Potent antimicrobial and antitumor activities have now been reported for NHC−metal complexes, specifically group 10 and 11 transition metal complexes.6 NHC ligands combine several features that make them useful for future development: the NHC backbone is modular and easily amenable to structural variations; their corresponding metal complexes are remarkably stable. In © 2012 American Chemical Society
particular, group 10 NHC chemistry is characterized by a kinetically robust M−C bond.4,5 Although the possibility to modulate their biological properties by derivatization of the Nheterocyclic carbene ligand has been addressed by several groups,7 the introduction of functionalities through the coordination of an additional relevant ligand has hardly been explored. The possibility to conjugate biomolecules including amino acids and small synthetic peptides to NHC−metal moieties could be exploited to generate hybrid cytotoxic species covalently linked to cell-directing agents as depicted in Scheme 1. Ligand substitution on a preformed NHC-complex precursor potentially represents a direct and practical method for the synthesis of such conjugates. However, due to the numerous neighboring heteroatom donor functionalities, the selective and controlled coordination of bioinspired molecules to the metal center still remains quite challenging.7c,8 We report herein a simple procedure for the preparation of NHC−platinum complexes that combine an amine, amino acid, or peptide as second neutral ligand in trans geometry. Antiproliferative activities of representative complexes were assayed against various cancer and healthy cell lines. Received: August 22, 2012 Published: October 18, 2012 7618
dx.doi.org/10.1021/om300806g | Organometallics 2012, 31, 7618−7621
Organometallics
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Scheme 1. Strategy of Functionalization of Pt−NHC Complexes with Bioinspired Molecules
Scheme 3. Scope of the Pyridine Substitution with Amino Acid and Oligopeptide Derivativesa
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RESULTS AND DISCUSSION We found that the labile character of pyridine induced by the trans effect of the carbene ligand 9,10 makes (NHC)PtX2(pyridine) complexes extremely valuable precursors to access functionalized PtII−NHC complexes (Scheme 2). NHCScheme 2. Pyridine Substitution with Diverse Amines and with Phenylhydrazine
a
PtI2(pyridine) complex 2 was readily prepared from imidazolium salt 1 in a one-step process in 72% yield.11 Addition of 1.2 equiv of simple primary and secondary amine models to platinum complex 2 led to the corresponding complexes in good to quantitative yield. This single step reaction provides a straightforward and high-yielding method to the synthesis of complex 3, which was previously obtained via a two-step method from a sensitive Karsted catalyst, as reported by Marinetti and co-workers.7b,d It is noteworthy that the displacement of pyridine by ethanolamine and phenylhydrazine proceeds chemoselectively. We further extended the pyridine substitution pathway to more complex amino-functionalized building blocks to create a diverse set of Pt−amine conjugates. Amino-ester derivatives, model dipeptides, and tripeptides were appended to the platinum center through the backbone amino group of the main chain. The complexes were obtained in one step from the corresponding ammonium salts of oligopeptides, whose side and terminal functional groups (i.e., carboxylic acids and amines) are orthogonally protected. Reaction with (NHC)PtI2(pyridine) 2 in the presence of a tertiary base (typically triethylamine) under heating at 55 °C in ethanol yielded the functionalized complexes 7−14 in yields ranging from 20% to 71% (Scheme 3). All complexes have been purified by column
Cl-Z: 2-chlorobenzyloxycarbonyl.
chromatography. The substitutions were highly selective12 since no additional products were recovered, thus indicating that the somewhat low yields observed may rather result from the low reactivity of the ligand itself.13 Figure 1 represents the molecular structure of the Pt−(S)prolinamide complex 7 that was determined by an X-ray diffraction study and that evidences the square-planar geometry around the metal center and the selective coordination of prolinamide through the N(3) atom. The carbene−Pt and Pt− N bond distances are 1.964(8) and 2.126(6) Å, respectively, which are in agreement with related complexes.7b The molecule crystallized in the chiral P212121 space group with one molecule of chloroform in the asymmetric unit. Thereby, only one diastereoisomer was observed in the solid state and the absolute configurations of C(12) and N(3) are S and R, respectively. Additionally, intra- and intermolecular hydrogen bonds between the N−H and the CO have also been evidenced in the crystal [H(3A)···O(1) distance of 2.119 Å and H(4C)···O(1) distance of 2.481 Å]. The proline torsion ϕ and ψ angles are −110° and +165°, respectively. Complex 11 also gave single crystals suitable for X-ray diffraction analysis. As shown in Figure 2, the crystal structure of 11 confirmed selective coordination of the terminal nitrogen 7619
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derivative, or tripeptide, were measured on a panel of different human tumor cell lines (MCF7, HCT116, PC3, and SK-OV3; see Table 1 for details) and on a noncancer cell line (MRC5).14 The half inhibitory concentrations (IC50) induced by these compounds are shown in Table 1. All complexes (entries 2−8) were found to be significantly more active than cisplatin (entry 1). The nature of the ligand was also found to influence the cytotoxic activity of the resulting complex. NHC platinum complexes bearing amino acid type ligands 7 and 9 (entries 5 and 6) display the highest potency against all cancer cells tested including cisplatin-resistant SK-OV3.15 Moreover, activities of complexes 7, 9, and 10 compare favorably with that of previously reported complex 3, bearing a simple cyclohexylamine ligand (entry 2).7b Interestingly, the use of ethanol in place of DMSO as solvent for the preparation of stock solutions does not affect the cytotoxic activity as displayed with complex 5 (entry 3 vs entry 4).
Figure 1. Molecular structure of Pt−(S)-prolinamide complex 7. Selected bond distances (Å) and angles (deg): Pt−C(1), 1.964(8); Pt−I(1), 2.6048(7); Pt−I(2), 2.5950(6); Pt−N(3), 2.126(6); C(16)− O(1), 1.22(1); C(16)−N(4), 1.36(1); C(12)−N(3), 1.484(9); H(3A)···O(1), 2.119(6); C(1)−Pt−N(3), 177.5(3); I(1)−Pt−I(2), 178.74(2); I(1)−Pt−C(1), 89.4(2); I(2)−Pt−N(3), 88.7(2); N(1)− C(1)−Pt−I(2), 83.4(6).
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SUMMARY In summary, we have demonstrated that benefits could be drawn from the pyridine lability trans to the NHC ligand in their corresponding Pt complexes to allow functionalization by metal coordination with diverse and sophisticated amines. The attachment of bioinspired molecules such as peptides has been successfully achieved, thus highlighting their potential as transition metal ligands. The selective coordination through the free amino group of the introduced ligand has been confirmed by single-crystal X-ray diffraction studies. The cytotoxic activities of these novel hybrid species against various human cancer cell lines have been demonstrated and compare well with that of cisplatin. We are currently further evaluating their biomedical properties varying the nature and the complexity of the grafted amine ligand (such as peptide-based cell-directing agents) in order to improve the cytotoxic profile and selectivity of the resulting complexes.
Figure 2. Molecular structure of Pt−peptide complex 11 [peptide = DVal-D-Phe-Gly-OMe]. Some hydrogen atoms are omitted for clarity.
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atom of the peptide and square-planar coordination at the metal center. Furthermore, within the cell, two conformational isomers are distinguishable and crystallized as H-bonded dimers stabilized by two intermolecular hydrogen bonds. In one molecule, the peptide backbone adopts a fully extended conformation with average backbone torsion angles ϕ and ψ of 150° and −156°, respectively. The second molecule adopts a more folded conformation with a ϕ angle for the residue coordinated to the metal (D-Val) of 71°. The average C−Pt and Pt−N bond lengths are 1.96 and 2.11 Å, respectively. To evaluate the biological effect of ligand variations trans to NHC, antiproliferative activities of representative complexes 3, 5, 7, 9, 10, and 12 with L = amino alcohol, amino acid
ASSOCIATED CONTENT
S Supporting Information *
Cif files for compounds 7 and 11 and full experimental procedures are available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Fax: +33 (0)388107246. Tel: +33 (0)388107166. E-mail:
[email protected]. Fax: +33 (0)540002200. Tel: +33 (0) 540003020. E-mail:
[email protected].
Table 1. Half Inhibitory Concentrations (IC50, in μM) of the Selected Compounds against Human Cancer Cell Lines (after 72h of incubation) compound
stock solution in
cisplatin 3 5 5 7 9 10 12
H2O DMSO DMSO EtOH DMSO DMSO DMSO DMSO
MCF7a 34.8 1.59 0.56 0.59 0.19 0.20 0.79 4.12
± ± ± ± ± ± ± ±
6.1 0.08 0.08 0.02 0.01 0.04 0.04 0.06
HCT116b 9.37 0.89 0.41 0.39 0.12 0.11 0.37 1.89
± ± ± ± ± ± ± ±
0.31 0.03 0.02 0.02 0.01 0.03 0.03 0.05
PC3c 15.0 1.99 0.82 0.84 0.34 0.30 0.91 4.57
± ± ± ± ± ± ± ±
0.7 0.18 0.04 0.05 0.06 0.08 0.08 0.90
SK-OV3d 16.4 1.36 1.07 0.57 0.17 0.23 0.63 3.68
± ± ± ± ± ± ± ±
2.6 0.22 0.27 0.06 0.01 0.16 0.01 0.02
MRC5e 29.0 1.24 0.43 0.31 0.15 0.13 0.42 2.09
± ± ± ± ± ± ± ±
2.1 0.02 0.01 0.05 0.01 0.05 0.01 0.29
a
MCF7, breast carcinoma. bHCT116, colon cancer cells. cPC3, prostate adenocarcinoma. dSK-OV3, human ovarian cancer cells. eMRC5, human fetus lung cells. 7620
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Notes
(12) In the particular case of complex 8, the selective coordination of the α-amino group over the guanidino group to platinum was easily evidenced by a COSY NMR experiment. This was also confirmed by the absence of reactivity when mixing complex 2 with Fmoc-Arg-OMe having a nonprotected lateral chain. (13) Addition of excess ligand leads to higher conversion, which is consistent with a low reactivity. (14) Due to the low solubilities of 3, 5, 7, 9, and 12 in water, we prepared and used stock solutions in DMSO. Since DMSO may react with the complexes to form mono-iodo−mono-DMSO adducts, the stability of the complexes in DMSO solution was investigated. No significant change was observed upon dissolution in DMSO, even at high temperatures. For example, after heating at 110 °C for 8 h, complex 3 was recovered and isolated in >80% yield. (15) Hills, C. A.; Kelland, L. R.; Abel, G.; Siracky, J.; Wilson, A. P.; Harrap, K. R. Br. J. Cancer 1989, 59, 527.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge the CNRS, the Ministère de l’Enseignement Supérieur et de la Recherche for a Ph.D. grant to E.C., and La Ligue contre le Cancer−Région Grand Est. The authors also thank Dr. B. Kauffmann and Dr. L. Brelot for X-ray diffraction studies. The crystallographic data have been collected at the IECB X-ray facility (UMS3033, CNRS, and Université de Bordeaux, US001 INSERM) and at the Institut de Chimie X-ray facility (UMR7177, CNRS, and Université de Strasbourg). Biological evaluations of cell proliferation inhibition have been performed at the Ciblothèque Cellulaire ICSN (Gif sur Yvette, France).
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